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Longest Common Prefix Matching | Set-6

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  • Difficulty Level : Easy
  • Last Updated : 12 May, 2021

Given a set of strings, find the longest common prefix.
Examples: 
 

Input: str[] = {geeksforgeeks, geeks, geek, geezer}
Output: gee

Input: str[] = {apple, ape, april}
Output: ap

 

Previous Approaches: Set1 | Set2 | Set3 | Set4 | Set5 
Approach: 
 

  • Sort the given set of N strings.
  • Compare the first and last string in the sorted array of strings.
  • The string with prefix characters matching in the first and last string will be the answer.

Below is the implementation of the above approach: 
 

C++




// A C++ Program to find the longest common prefix
#include <bits/stdc++.h>
using namespace std;
 
// A Utility Function to find the common prefix between
// first and last strings
string commonPrefixUtil(string str1, string str2)
{
    string result;
    int n1 = str1.length(), n2 = str2.length();
 
    // Compare str1 and str2
    for (int i = 0, j = 0; i <= n1 - 1 && j <= n2 - 1; i++, j++) {
        if (str1[i] != str2[j])
            break;
        result.push_back(str1[i]);
    }
 
    return (result);
}
 
// A Function that returns the longest common prefix
// from the array of strings
void commonPrefix(string arr[], int n)
{
    // sorts the N set of strings
    sort(arr, arr + n);
 
    // prints the common prefix of the first and the
    // last string of the set of strings
    cout << commonPrefixUtil(arr[0], arr[n - 1]);
}
 
// Driver Code
int main()
{
    string arr[] = { "geeksforgeeks", "geeks",
                     "geek", "geezer" };
    int n = sizeof(arr) / sizeof(arr[0]);
 
    commonPrefix(arr, n);
 
    return 0;
}

Java




// A Java program to find the longest common prefix
 
import java.util.Arrays;
 
class GFG {
 
// A Utility Function to find the common prefix between
// first and last strings
    static String commonPrefixUtil(String str1, String str2) {
        String result = "";
        int n1 = str1.length(), n2 = str2.length();
 
        // Compare str1 and str2
        for (int i = 0, j = 0; i <= n1 - 1 && j <= n2 - 1; i++, j++) {
            if (str1.charAt(i) != str2.charAt(j)) {
                break;
            }
            result += str1.charAt(i);
        }
 
        return (result);
    }
 
// A Function that returns the longest common prefix
// from the array of strings
    static void commonPrefix(String arr[], int n) {
        // sorts the N set of strings
        Arrays.sort(arr);
 
        // prints the common prefix of the first and the
        // last String of the set of strings
        System.out.println(commonPrefixUtil(arr[0], arr[n - 1]));
    }
 
// Driver Code
    public static void main(String[] args) {
        String arr[] = {"geeksforgeeks", "geeks",
            "geek", "geezer"};
        int n = arr.length;
 
        commonPrefix(arr, n);
 
    }
}
/* This JAVA code is contributed by 29AjayKumar*/

Python3




# A Python 3 Program to find the
# longest common prefix
 
# A Utility Function to find the common
# prefix between first and last strings
def commonPrefixUtil(str1, str2):
    n1 = len(str1)
    n2 = len(str2)
     
    result = ""
     
    # Compare str1 and str2
    j = 0
    i = 0
    while(i <= n1 - 1 and j <= n2 - 1):
        if (str1[i] != str2[j]):
            break
        result += (str1[i])
         
        i += 1
        j += 1
 
    return (result)
 
# A Function that returns the longest
# common prefix from the array of strings
def commonPrefix(arr, n):
     
    # sorts the N set of strings
    arr.sort(reverse = False)
 
    # prints the common prefix of the first
    # and the last string of the set of strings
    print(commonPrefixUtil(arr[0], arr[n - 1]))
 
# Driver Code
if __name__ == '__main__':
    arr = ["geeksforgeeks", "geeks",
                    "geek", "geezer"]
    n = len(arr)
 
    commonPrefix(arr, n)
     
# This code is contributed by
# Sanjit_Prasad

C#




// C# Program to find the longest
// common prefix
using System;
 
class GFG
{
// A Utility Function to find the common
// prefix between first and last strings
static String commonPrefixUtil(String str1,
                               String str2)
{
    string result = "";
    int n1 = str1.Length, n2 = str2.Length;
     
    // Compare str1 and str2
    for (int i = 0, j = 0;
             i <= n1 - 1 && j <= n2 - 1; i++, j++)
    {
        if (str1[i] != str2[j])
            break;
        result += (str1[i]);
    }
     
    return (result);
}
 
// A Function that returns the longest
// common prefix from the array of strings
static void commonPrefix(String []arr, int n)
{
    // sorts the N set of strings
    Array.Sort(arr);
 
    // prints the common prefix of the first
    // and the last String of the set of strings
    Console.Write(commonPrefixUtil(arr[0],
                                   arr[n - 1]));
}
 
// Driver Code
public static void Main()
{
    String []arr = {"geeksforgeeks", "geeks",
                    "geek", "geezer"};
    int n = arr.Length;
    commonPrefix(arr, n);
}
}
 
// This code is contributed by 29AjayKumar

PHP




<?php
// A PHP Program to find the longest
// common prefix
 
// A Utility Function to find the common
// prefix between first and last strings
function commonPrefixUtil($str1, $str2)
{
    $result = "";
    $n1 = strlen($str1);
    $n2 = strlen($str2);
 
    // Compare str1 and str2
    for ($i = 0, $j = 0; $i <= $n1 - 1 &&
          $j <= $n2 - 1; $i++, $j++)
    {
        if ($str1[$i] != $str2[$j])
            break;
        $result = $result.$str1[$i];
    }
 
    return ($result);
}
 
// A Function that returns the longest
// common prefix from the array of strings
function commonPrefix(&$arr, $n)
{
    // sorts the N set of strings
    sort($arr);
 
    // prints the common prefix of the first
    // and the last string of the set of strings
    echo commonPrefixUtil($arr[0], $arr[$n - 1]);
}
 
// Driver Code
$arr = array("geeksforgeeks", "geeks",
             "geek", "geezer" );
$n = sizeof($arr);
 
commonPrefix($arr, $n);
 
// This code is contributed by ita_c
?>

Javascript




<script>
// A Javascript program to find the longest common prefix
     
     
    // A Utility Function to find the common prefix between
// first and last strings
    function commonPrefixUtil(str1,str2)
    {
        let result = "";
        let n1 = str1.length, n2 = str2.length;
   
        // Compare str1 and str2
        for (let i = 0, j = 0; i <= n1 - 1 && j <= n2 - 1; i++, j++) {
            if (str1[i] != str2[j]) {
                break;
            }
            result += str1[i];
        }
   
        return (result);
    }
     
    // A Function that returns the longest common prefix
// from the array of strings
    function commonPrefix(arr,n)
    {
        // sorts the N set of strings
        arr.sort();
   
        // prints the common prefix of the first and the
        // last String of the set of strings
        document.write(commonPrefixUtil(arr[0], arr[n - 1])+"<br>");
    }
     
    // Driver Code
     
    let arr=["geeksforgeeks", "geeks",
            "geek", "geezer"];
     
    let n = arr.length;
     
    commonPrefix(arr, n);
             
     
     
 
// This code is contributed by avanitrachhadiya2155
</script>

Output: 

gee

 

Time Complexity: O(N * log N)
 


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